Fluid control devices include various categories of equipment including control valves and regulators. Such control devices are adapted to be coupled within a fluid process control system such as chemical treatment systems, natural gas delivery systems, etc., for controlling the flow of a fluid therethrough. Each control device defines a fluid flow-path and includes a control member for adjusting a dimension of the flow-path. For example, FIG. 1 depicts a known control valve regulator assembly 10 including a valve body 12 and an actuator 14. The valve body 12 defines a flow-path 16 and includes a throat 18. In FIG. 1, the regulator assembly 10 is configured in a flow-up configuration. The actuator 14 includes an upper actuator casing 20, a lower actuator casing 22, a diaphragm subassembly 30 including a diaphragm 32, and a control member 24.
The control member 24 is disposed within the upper and lower actuator casings 20, 22 and is adapted for bi-directional displacement in response to changes in pressure across the diaphragm subassembly 30. So configured, the control member 24 controls the flow of fluid through the throat 18. In some applications, a retainer 31 is secured to the bottom of the control member 24. The retainer 31 secures a sealing element 28 to the bottom of the control member 24. The retainer 31 includes a lower surface 33, distal to the control member 24 that has a generally convex shape, which extends outwardly beyond the sealing element 28, and past which the fluid flows when the regulator assembly 10 is in the open position. Additionally, the regulator assembly 10 includes a seat ring 26 disposed in the throat 18 of the valve body 12. When the outlet pressure of the valve body 12 is high, the sealing element 28 of the control member 24 may sealingly engage the seat ring 26 and close the throat 18. Similarly, absent any pressure in the actuator 14 or upon the failure of the diaphragm 32, a coil spring 34 disposed within an annular cavity portion 36 of the upper actuator casing 20 biases the control member 24 into the closed position. Such a regulator is commonly known as a “fail close” regulator. “Fail open” regulators operate similar to “fail closed” regulators; however, upon failure of the diaphragm, a spring biases the control member open, rather than closed.
Turning now to FIG. 2, the retainer 31 is illustrated in more detail. The retainer 31 is secured to the bottom of the control member 24 by a plurality of fasteners 35. The sealing element 28 is disposed between an upper surface 37 of the retainer 31 and a covering flange 39, which extends radially outward from the control member 24. The bottom surface 33 of the retainer 31 includes a first portion 33a, proximate a longitudinal axis A of the control member 24, the first portion 33a extending perpendicular to the longitudinal axis A of the control member 24 for a distance and a second portion 33b that angles radially upward, towards the sealing element 28. Generally speaking, the second portion 33b is angled about 30 degrees with respect to the longitudinal axis A. A balance cavity 41 may extend through the retainer 31 in some cases where a balanced valve is desired. When a balance cavity 41 extends through the retainer, the first portion 33a may extend outwardly from an edge of the balance cavity 41. The balance cavity 41 is a cylindrical opening having a longitudinal axis that is coincident with the longitudinal axis A of the control member 24. In other words, the retainer 31 has a longitudinal axis that is coincident with the longitudinal axis A.
Generally, valve sizing for a particular operation is determined such that the control member 24 will normally operate in the middle 80% of control member 24 travel for a majority of the operational time. By operating in the middle 80%, flow between the seat ring 26 and the sealing element 28 is predictable and smooth. However, in some cases, where a properly sized valve is not available, a valve that is larger than optimum may be used. In this case, the control member 24 and thus the sealing element 28 may operate a majority of the time relatively close to the seat ring 26, for example, within the closest 20% of the control member 24 travel to the seat ring 26, which can cause relatively high fluctuating flow velocities between the sealing element 28 and the seat ring 26 due to the formation of restricted flow areas. These high flow velocities can damage the relatively soft sealing element 28, which affects the shutoff capability of the control member 24.